Here are the major in-game cheat codes available for Duke Nukem 3D. Type these codes (the part in bold) in while you're playing the game to activate the cheats:

dncoords: Display your map position
dncornholio or dnkroz: Toggle God mode
dncashman: Press Spacebar to get money
dnclip: Walk through some walls
dndebug: Display debug information
dnending: End episode
dnhyper: Unlimited steroids
dninventory: All weapons, ammo & items
dnitems: Maximum armor, all keys
dnkeys: All keys
dnmonsters: Toggle monsters
dneat: Full health
dnnuk:Full ammo
dnrate: Display frame rate
dnscotty[episode#][level#]: Go to another level
dnshowmap: Display entire map
dnskill[skill level, 0-4]: Change skill level
dnstuff: All weapons, maximum ammo, all keys
dnunlock: Toggle all doors
dnview: 2nd viewpoint
dnweapons: All weapons and ammo

Does blowing away pigcops, destroying entire buildings, and shooting your way through an adult XXX store sound enjoyable? Then you'll love Duke 3D. (Note: There isn't actually anything X-rated in the game.) Uses the "Build" engine, which is still similar to the old Wolf3D engine in that there are no rooms on top of other rooms, but it's been tweaked and pushed to the max. Lots of fun! (Note: Several people have emailed me to comment that there ARE rooms-above-rooms in Duke3D. While the effect of rooms-above-rooms can be _simulated_ in Build, there are never really any rooms on top of other rooms in the game, and thus it's not truly 3D.)